U-value & Glaser interstitial condensation check for walls, floors & roofs — layers listed interior → exterior (cold side)
| Layer name | Thickness (mm) | λ (W/mK) | µ (Mu) | R (m²K/W) | Sd (m) | Risk | Vent. |
|---|
Vent. column — Exterior (→ Ext.): Layer is open to outside air (cladding battens, rain-screen, perforated facade). Vapour pressure resets to exterior conditions pout; outer layers carry no condensation risk. Per BS EN ISO 6946:2017 Annex B. | Interior (← Int.): Layer is open to room air (service void, plasterboard-on-battens, floor void). Temperature ≈ room temp; vapour pressure resets to interior conditions pin. Inner layers between the room and this void carry no vapour gradient — but layers outboard of it face full interior humidity, which may increase risk.
Winter check vs annual drying: this page is a worst-case design-day snapshot — it flags whether condensation could form under the single cold condition you set. The Annual drying analysis integrates 12 months of mean climate and credits summer drying, answering instead "does moisture accumulate over a year?" A layer can flag here yet still dry out across the year (or, because the annual tool uses monthly means, pass annually while still condensing on a true cold snap). Read them together.
Annual drying analysis →X-axis: physical thickness (mm). Temperature and vapour pressure curves steepen through insulating/resistive layers and flatten through dense conductive ones — this is physically correct. The purple dashed line shows the local dew point temperature at each position through the build-up: where the blue temperature line crosses below it, condensation occurs. Condensation zones are shaded red.
| Term | Symbol / unit | What it means |
|---|---|---|
| Thermal conductivity | λ (W/mK) | How readily a material conducts heat. Lower = better insulator. Mineral wool ≈ 0.035; concrete ≈ 1.35; still air ≈ 0.025. |
| Thermal resistance | R (m²K/W) | Resistance of a specific layer to heat flow. R = thickness (m) ÷ λ. Layers add in series. Higher = better. |
| U-value | U (W/m²K) | Heat loss rate per m² per degree of temperature difference. U = 1 ÷ total R (including surface resistances). Lower = better. UK Part L targets: walls ≤ 0.18, floors ≤ 0.13, roofs ≤ 0.11. |
| Vapour diffusion resistance factor | µ (Mu) — dimensionless | How many times more resistant to vapour diffusion a material is compared to the same thickness of still air. µ = 1: same as air (mineral wool, open-cell foam). µ = 10: plasterboard, brick. µ = 100–200: dense concrete, cement render, smart VCL. µ = 100,000+: polythene sheet. |
| Diffusion-equivalent air layer | Sd (m) | The thickness of still air that would offer the same vapour resistance as the layer: Sd = µ × thickness (m). A 10 mm cement render at µ = 200 gives Sd = 2 m — the vapour resistance of 2 metres of air. |
| Exterior ventilated cavity (→ Ext.) | — | An air gap open to exterior air — cladding battens, open-joint facades, perforated rain-screen. Never airtight; vapour cannot accumulate. Vapour pressure resets to pout here; all outer layers have no condensation risk. Per BS EN ISO 6946:2017 Annex B. |
| Interior ventilated void (← Int.) | — | A gap open to interior room air — service void, plasterboard-on-battens without airtight seal, floor/ceiling void within the thermal envelope. Temperature ≈ room temperature; vapour pressure ≈ pin throughout. Inner layers (between room and void) carry no vapour gradient. Layers outboard of the void face the full interior vapour pressure — increasing condensation risk compared to a sealed assembly. A VCL must be placed on the warm side of the insulation after this void to be effective. |
| Dew point | °C | The temperature at which air becomes saturated and vapour condenses. If the temperature at any point in the build-up falls below the dew point of the vapour at that point, liquid water forms there. |
| Saturation vapour pressure | Psat (Pa) | The maximum vapour pressure air can hold at a given temperature (Magnus formula). Drops sharply as temperature falls. Condensation occurs where actual vapour pressure exceeds Psat — shown as the red risk zone in the Glaser diagram. |
| Surface resistance | Rsi / Rse (m²K/W) | The thermal resistance of the boundary air films on interior (Rsi) and exterior (Rse) surfaces. Per BS EN ISO 6946 the interior film depends on heat-flow direction: walls (horizontal) Rsi = 0.13, roofs (upward) Rsi = 0.10, floors (downward) Rsi = 0.17. Rse = 0.04 for surfaces exposed to external air; sheltered or below-grade locations use Rse = 0.13. The element-type selector sets these automatically. |
| Glaser method | — | A steady-state check for interstitial condensation risk defined in BS EN ISO 13788. It assumes constant winter conditions and does not model seasonal drying. It is conservative. For below-grade assemblies or moisture-critical details, use WUFI (dynamic hygrothermal simulation) for a more accurate picture. |
| Reference | Used for / notes |
|---|---|
| BS EN ISO 10456:2007 | Building materials and products — Hygrothermal properties — Tabulated design values. Primary source for λ and µ defaults for most materials in this tool. Tables 1–4 cover insulation products, masonry, timber, renders, and membranes. iso.org |
| BS EN ISO 6946:2017 | Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods. U-value method (R = d/λ, U = 1/ΣR), standard surface resistances (Rsi = 0.13, Rse = 0.04/0.13), and treatment of unventilated and ventilated air layers (Annex B). Air gap R-values approximate Table B.1 using λ = 0.17 W/mK at 25 mm (R ≈ 0.147, cf. standard value 0.15 m²K/W). |
| BS EN ISO 13788:2012 | Hygrothermal performance of building components — Internal surface temperature to avoid critical surface humidity and interstitial condensation. Defines the Glaser method used here: steady-state vapour diffusion (Sd = µ × d), saturation vapour pressure via the Magnus equation, and the condensation criterion (pv ≥ psat). |
| BRE Report BR 443 (2019 ed.) | Conventions for U-value calculations. Building Research Establishment. Cross-reference for surface resistance conventions and worked examples for UK practice. bre.co.uk |
| CIBSE Guide A (2015) | Environmental Design. Chartered Institution of Building Services Engineers. Section 3 provides thermal properties of building elements. Cross-reference for brick, concrete, render, and air gap λ values. |
| Building Regulations Part L (England, 2021) | U-value target bands in the results panel are selected by the chosen element type (Approved Documents L1A/L2A, effective June 2022, England): wall ≤ 0.18 new-build / 0.28 notional / 0.35 retained; roof ≤ 0.11 pitched / 0.15 flat / 0.16 retained; floor ≤ 0.13 new-build / 0.18 limiting / 0.25 retained. Surface resistances are set from BS EN ISO 6946 by heat-flow direction (wall 0.13/0.04, roof 0.10/0.04, floor 0.17/0.04). Ground-bearing floors are simplified — full ground coupling requires BS EN ISO 13370. Note: Scotland (Section 6), Wales (Part L), and Northern Ireland (Technical Booklet F) have differing requirements. |
| Material-specific notes |
Sandstone (λ = 2.30): ISO 10456 Table 3 gives 2.3 for medium-density sandstone; lower-density/porous types can be as low as 0.7 — verify for specific stone. Fibre cement panel (λ = 0.58, µ = 35): Manufacturer data (Marley Eternit, Equitone). No ISO 10456 tabulated value; verify against product BBA certificate. Cementitious tanking render (µ = 200): No ISO 10456 tabulated value; typical product range 100–300 (Sika, Vandex, Xypex datasheets). Verify against BBA certificate or test data. Smart VCL / Pro Clima Intello (µ = 5000 ≈ Sd 1.0 m): Hygrovariable membrane — its Sd is set by the local humidity at the membrane plane: ≈ 0.25 m when humid (summer: vapour-open, lets the structure dry inward), rising to ~10 m only when very dry. Behind a heated room in winter the local RH is ~50%, so it settles around Sd ≈ 1 m — the default here. That is roughly 20× more vapour-open than a sealed polythene VCL (Sd ≈ 20 m), so a smart membrane gives markedly less winter outward protection; its safety relies on summer inward drying, which steady-state Glaser cannot represent. Consequently this tool may predict winter condensation against a cold vapour-resistant sheathing (e.g. OSB — see note below) behind a smart VCL even where the assembly is sound across a full year. For such build-ups use a vapour-open sheathing, or assess with the Annual drying analysis (BS EN ISO 13788 §6) or WUFI. Tune µ per project: µ = Sd ÷ 0.0002 at 0.2 mm. (The earlier µ = 150 default gave Sd 0.03 m — more open than even the summer state, and a bug.) OSB/3 — dry-cup µ50 / wet-cup µ30 (Sd 0.60 / 0.36 m at 12 mm): OSB is itself mildly hygrovariable (a weak "smart" vapour retarder): its vapour resistance falls as humidity rises, so it gets more vapour-open when wet. EN 13986 / ISO 10456 tabulate µ = 50 (dry-cup, 0–50% RH) and µ = 30 (wet-cup, 50–100% RH); the Norbord/West Fraser Sterling OSB/3 BBA cert cites the same. For a winter condensation check the OSB sits cold with high local RH, so the wet-cup value (µ ≈ 30, Sd ≈ 0.36 m) is the more representative preset — a single dry-cup value over-states its tightness and over-predicts condensation. Caveats: independently measured dry-state Sd is often far higher and varies widely by product (Ojanen & Ahonen 0.38→2.43 m; Korsnes µ ≈ 110–470), and permeability rises with wetting history — so use product-specific test data where it matters, and assess OSB-behind-smart-VCL walls by annual drying (WUFI / BS EN 15026), not the steady-state flag. (Refs: Korsnes, Passivhus Norden 2013; Ojanen & Ahonen, VTT 2005; West Fraser/APA J450; GreenBuildingAdvisor; Pro Clima.) External air barriers — Proctor Wraptite (Sd 0.039 m) & Pro Clima Solitex Adhero 3000 (Sd 0.40 m): Self-adhesive, monolithic, vapour-permeable membranes for the outer face of the sheathing (airtight on the cold side). They are vapour-open, not hygrovariable — Sd is fixed, not RH-variable like a smart VCL. Note the two are an order of magnitude apart: Wraptite (0.039 m) is near-transparent to vapour; Adhero 3000 (µ 570 × 0.7 mm = 0.40 m) is only moderately open. Placed outboard of OSB they do not relieve the cold-OSB condensation flag — the OSB inboard of them stays the bottleneck, and because they add resistance on the outer side they keep the plane's vapour pressure the same (Wraptite) or raise it (Adhero 3000 can make a steady-state result worse). Their real value is external airtightness + outward drying: pair an external air barrier with a vapour-open sheathing (drop the OSB) so the wall dries outward — that is what clears the flag. Sources: A. Proctor Group Wraptite datasheet (Sd 0.039 m); Pro Clima Solitex Adhero 3000 datasheet (Sd 0.40 m, µ 570, g 2.0 MN·s/g). Closed-cell spray foam (µ = 150): Aged in-situ λ typically rises to 0.030–0.035 from declared 0.028. Use declared value for new installations. Rammed / compressed earth (λ = 1.10, µ = 20): ISO 10456 gives 1.1 for unfired clay; actual values depend heavily on soil composition and moisture content — treat as indicative. |
| Alternative open sources |
ASHRAE Fundamentals Handbook — US equivalent; broadly consistent with ISO 10456 but uses imperial units. Fraunhofer IBP WUFI database — extensive hygrothermal property database, freely browsable at wufi.de. BBA / UKAS certificates — product-specific certified λ and µ data for UK-approved products at bbacerts.co.uk. Specific Heat Capacity (cp): ISO 10456 also tabulates cp (J/kgK) for dynamic simulation (WUFI, EnergyPlus) — not used in this steady-state tool. |
| Limitations of this tool | Steady-state Glaser analysis is conservative. It does not model: (1) seasonal drying capacity, (2) capillary redistribution, (3) hygroscopic buffering, or (4) solar-driven vapour. For any assembly where risk is predicted — particularly below-grade, warm roofs, or highly insulated retrofits — validate with WUFI Pro or commission a BS EN 15026 dynamic simulation. This tool is suitable for initial feasibility and education; not for Building Regulations compliance submissions without further assessment. |